Automatically shiftable transmissions used in transportation vehicles, such as cars, buses, and trucks, require a positive displacement pump to supply pressurized hydraulic fluid for engagement of clutches and brakes, torque converter operation, and cooling.
These pumps require power from the engine or prime mover to supply the required control pressure. The power absorbed by the pump and therefore supplied by the engine is a function of the pressure and displacement of the pump. The higher the pump output pressure or main pressure of the transmission, the more horsepower required from the engine.
Current transmissions utilize control mechanisms having electronic systems. These electronic systems are supplied with signals from the engine, vehicle, and transmission. The signals are utilized to determine the operation of various solenoid valves within the control system to modulate various pressures including the main pressure or line pressure of the transmission. By modulating the main pressure, the fuel economy of the vehicle may be improved.